Evolution of the current data transport networks is hindered by a number of constraints. For example, the optical transport networks operate according to fiber specific transmission protocols, each having different levels of operation, administration, maintenance and provisioning (OAM&P) functionality, so that the nodes must be equipped with protocol-specific hardware and software. In addition, handling a plurality of protocols at speeds over 100 Mb/s poses real problems for the current generation of microprocessors.
One way to increase the speed (and bandwidth) of the network is to replace the electronics components with optical components. The increased transport capacity requirements are also met by the introduction of point to point optical fiber systems, carrying TDM signals. An example of a TDM network is SONET/SDH, which transports hierarchically multiplexed lower rate tributaries into a higher rate TDM signal. SONET/SDH is a physical layer technology which is currently used as a transport service for ATM, SMDS, frame relay, T1, E1, etc. SONET/SDH provides the ability to combine and consolidate traffic from different locations through one facility (grooming) and reduces the amount of back-to-back multiplexing. More importantly, network providers can reduce the operation cost of their transmission network by using the comprehensive OAM&P features of SONET.
Originally, optical transport networks were intended to be bit-rate and data format independent, thus providing for the transportation of a wide variety of data signals. Unfortunately, current, optical transport networks, including SONET/SDH networks, do not achieve this goal, as the line rates for these networks have been restricted to a set of discrete transmission rates. Similarly, current electrical transport networks, such as the DS3 electrical network, are also limited to particular, discrete transmission rates. Thus, data characterized by a transmission rate that does not belong to the set of pre-defined transmission rates is not directly transportable over such data networks. In many cases, a user signal must undergo a mapping operation to be able to be transported by the data network.
The mapping of one rate or format into another is well known. For example, Bellcore TR-0253 describes in detail the standard mappings of the common asynchronous transmission formats (DS0, DS1, DS2 and DS3, among others) into SONET. Similar mappings are defined for the ETSI hierarchy mapping into SDH.
Unfortunately, the standards or proprietary schemes allow the transportation of only a very specific set of signals, with format specific hardware. Thus, these methods of mapping cannot be used to map rates that vary significantly from the standard. Furthermore, these mappings are each precisely tuned for a particular format and a particular bit-rate, with for example a ±20 ppm (parts per million of the bit rate) tolerance. If a signal has, for example, a bit rate even 1% different than that of a DS3, it cannot be transported over a SONET/SDH network. In addition, a different hardware unit is generally required to perform the mapping of each kind of signal.
In a somewhat different approach discussed in the U.S. patent application Ser. No. 09/349,087 (Roberts), entitled “Mapping Arbitrary Signals into SONET”, filed on Jul. 8, 1999 and assigned to Nortel Networks Limited, arbitrary electrical signals are converted into SONET optical signals by using a synchronizer. The synchronizer maps the arbitrary electrical signals into SONET signals such that the electrical signals can be recovered with low timing jitter at low cost at the far end. This mapping method can be used for tributaries of almost any continuous format. The synchronizer recognizes selected protocols, frames on them, and effects the corresponding performance monitoring.
Also, U.S. patent application Ser. No. 09/438,516 (Roberts et al.), entitled “Detection of Previous Section Fail for a Transparent Tributary”, filed on Nov. 12, 1999 and assigned to Nortel Networks Limited, discloses a method and apparatus for transmitting a continuous digital signal of an arbitrary rate R1 over a synchronous network as a transparent tributary. The rate R1 of the continuous digital signal is detected and a protocol corresponding to the rate R1 is determined, according to which protocol a set of performance parameters on the continuous digital signal are measured and reported.
The background information provided above indicates that there is a need in the industry to improve the technology for transparently transmitting electrical data signals, in particular signals of arbitrary transmission rate, over a data network.